1. Field of the Invention
The present invention relates to a pixel structure of display, and more particularly, to a pixel structure capable of improving the color-shifting of display.
2. Description of the Related Art
Due to the advantages of light weight, thinness depth, small volume, and lower radiation, the liquid crystal displays (LCD) of the flat-panel display whose display effect is much superior to that of a cathode ray tube (CRT) display has attracted the public interest in recent years. The consumers also request the preferably images displayed in the LCD.
According to the propagation direction of the ray manner, LCD can be categorized into three types: transmissive type, reflective type, and transflective type. In the transmissive type LCD, the light source is provided by a backlight source, and has the advantages of good image display under the environment having normal light and the dark. However, it is difficult to clearly view or to recognize the image display of the transmissive type LCD under the sunlight (for example, the user want to use the LCD in outdoors). In the reflective type LCD, ambient light is used as the light source (i.e. no backlight system), so that good image display is presented in indoors filled with light or outdoors. Also, the power consumption of the reflective type LCD is lower than that of the transmissive type LCD. The transflective type LCD, possessing the advantages of the transmissive type and reflective type LCDs, has been applied in the portable electronic products such as cellular phone, personal digital assistant (PDA), etc.
In general, a LCD is assembled by an upper substrate and a lower substrate. The space between the upper substrate and the lower substrate is filled with liquid crystal layer having numerous LC molecules. The polarization direction of the light passing through the liquid crystal layer is modulated by changing the arrangement direction (i.e. alignment direction) of the liquid crystal molecules that is varying with a voltage applied to the pixel electrode. In this way, the polarized reflected light has the brightness corresponding to the voltage applied to the pixel electrode. When a voltage is applied to the pixel electrodes, the arrangement direction of the liquid crystal molecules is to be varied so that the light transmission changes. Thus, the LCD can display images with different brightness such as white, black, and the different gray scale, in which including the intermediate of the gray scale. In addition, the liquid crystal molecules of the LCD can be categorized into twisted nematic (TN) mode and vertical alignment (VA) mode. When a voltage is not applied to the pixel electrodes, the TN mode liquid crystal molecules gradually twist layer by layer from one of the liquid crystal molecules of the substrates to another of the liquid crystal molecules of the substrates having a angle, for example, the uppermost layer of the liquid crystal molecules near the upper substrate to the bottom layer of the liquid crystal molecules near the lower substrate having a 90° angle. When a sufficient voltage is applied, the TN mode liquid crystal molecules are to be aligned and parallel to the direction of the electric field. The VA mode liquid crystal molecules, differently, are aligned and perpendicular to the upper and lower substrates when a voltage is not applied, and are twisted a 90° angle to be aligned and parallel to the upper and lower substrates when a sufficient voltage is applied.
For an LCD panel with a large size, such as panel used in a notebook, multi-domains in every pixel of the panel are formed to make high resolution and wide viewing angle of LCD displays.
FIG. 1A and FIG. 1B illustrate the arrangement of multi-domains liquid crystal molecules in vertical alignment mode of an LCD panel when a voltage is applied and not applied to the panel, respectively. The upper substrate structure (i.e. the first substrate structure) 10 and the lower substrate structure (i.e. the second substrate structure) 20 are assembled in parallel and the space between them is filled with a liquid crystal layer 30 containing numerous liquid crystal molecules 302. The lower substrate structure 20 includes a substrate (such as a glass substrate) 202 on which a thin film transistor (TFT), the metal layer(s), and the insulating layer(s) (those device and layers not being shown in figures) are formed. A pixel electrode 204 is disposed above the insulating layer and is covered with an alignment film 206. As shown, each of the pixel electrodes 204 is isolated with the spacing 208, and the bottoms of the spacings 208 are covered with the alignment film 206. The upper substrate structure 10 includes a first substrate (such as a glass substrate) 102, a transparent electrode (such as ITO electrode) 104, and an alignment film 106. Also, a protrusion 108 is formed at the upper substrate structure 10 and is covered with the alignment film 106.
As shown in FIG. 1A, when no voltage is applied to the panel, most of the liquid crystal molecules 302 are aligned vertically to the pixel electrode 204. The liquid crystal molecules 302 adjacent to the protrusion 108 are arranged substantially vertical to the surfaces of the protrusion 108, and have an inclination to the pixel electrode 204. Thus, the protrusion 108 provides a pre-tilt angle for the liquid crystal molecules 302 while no voltage is applied.
As shown in FIG. 1B, when a voltage is applied to the panel, two different domains are formed in a single pixel because of the different inclinations of the molecules 302 on the left and right sides of the protrusion 108. To be more specific, the electric field affects the LC molecules, so as to let the LC molecules adjacent to the left side of the protrusion 108 affect the left portion of the liquid crystal molecules 302 of the pixel, so that the left portion of LC molecules incline towards the right side. Likewise, the electric field affects the LC molecules, so as to let the LC molecules adjacent to the right side of the protrusion 108 affect the right portion of the liquid crystal molecules 302 of the pixel, resulting in the inclination of right portion of LC molecules towards the left side. FIG. 1A and FIG. 1B show the example with only two domains in one single pixel. By changing the shape of the protrusion 108, multiple domains can be similarly implemented, leading to a wide viewing angle of display. However, the protrusion 108 can easily cause the problem of light leaking.
Besides formation of the protrusions 108, multiple domains can be achieved by forming the slits at the pixel electrode. Each pixel area can be divided into several domains by the slits. When a voltage is applied to the panel, a slanted electric field is generated adjacent to the edges of the pixel electrode (cut by the slits) so as to cause the inclination of LC molecules near the slits. Those inclined LC molecules affect the other LC molecules, so that multi-domains within a pixel and wide viewing angle of display can be obtained.
Referring to FIG. 2A, FIG. 2B and FIG. 2C, for illustrating the substrate having TFT structure. FIG. 2A is a cross-sectional view taken along the line 2A-2A of FIG. 2C illustrating a thin film transistor (TFT) of a second substrate (lower substrate) structure of LCD. FIG. 2B is a cross-sectional view taken along the line 2B-2B of FIG. 2C illustrating a storage capacitor (CST) of a second substrate structure of LCD. FIG. 2C schematically illustrates a single sub-pixel of a multi-domain vertical alignment (MVA) mode of the TFT-LCD. Also, the TFT-LCD shown in FIG. 2C is a “CST on common” (i.e. storage capacitors on a common electrode) design.
A conventional TFT-LCD is assembled by a first substrate structure (or upper substrate structure) and a second substrate structure (or lower substrate structure). The first substrate structure comprises a number of transparent pixel electrodes, color filters, and black matrices. The second substrate structure comprises a number of scan lines, data lines, storage capacitors, switching elements (e.g., TFTs), and transparent pixel electrodes. In the TFT-LCD, the data lines perpendicularly intersect the scan lines to form a number of pixel regions. In a full-color LCD display, each pixel consists of three sub-pixels: red, green, and blue (RGB) sub-pixels, and each sub-pixel is controlled by a TFT. Also, each sub-pixel region is defined by a pair of scan lines and the corresponding data lines. Each sub-pixel region includes a storage capacitor CST, a TFT, and a pixel electrode (e.g. a transparent ITO). FIG. 2C can be represented as one of single R, G, or B sub-pixel of full-color displays.
As shown in FIG. 2A (taken along the cross-sectional line 2A-2A of FIG. 2C), the second substrate structure comprises a second substrate 202 and a gate electrode 212 is formed (by patterning a first metal layer) on the second substrate 202. A first insulating layer 213 is formed on the second substrate 202 and covers the gate electrode 212. An amorphous-Si (a-Si) layer is formed on the first insulating layer 213 and then patterned to form a channel 215. Drain (D) and source (S) are formed on the first insulating layer 213, by patterning a second metal layer. Next, a passivation layer 216 is formed on the drain (D) and the source (S) and covers the first insulating layer 213. A contact hole 217 is then formed within the passivation layer 216 to expose the partial surface of source (S)/drain (D). Finally, a pixel electrode (e.g. transparent ITO) 204 is formed on the passivation layer 216 and fills the contact hole 217, so that the pixel electrode 204 is coupled to source (S)/drain (D).
The scan lines and data lines are respectively formed during the patterning step of forming the gate electrode 212 and source (S)/drain (D), respectively. Also, the scan lines and data lines are isolated by the first insulating layer 213.
As shown in FIG. 2B (taken along the cross-sectional line 2B-2B of FIG. 2C), a storage capacitor (CST) includes a common electrode 214 and a capacitor electrode 218. The common electrode 214 and the capacitor electrode 218 are separated by the first insulating layer 213. The storage capacitor (CST) is formed together with the formation of the TFT. The common electrode 214 is formed after the formation and patterning of the first metal layer. Likewise, after the formation and patterning of the second metal layer, the capacitor electrode 218 is formed. The passivation layer 216 covers the capacitor electrode 218 and the first insulating layer 213. A contact hole 219 is further formed within the passivation layer 216. When the pixel electrode 204 is formed over the passivation layer 216, the pixel electrode 204 and the capacitor electrode 218 are electrically coupled through the contact hole 219. In addition, all of the common electrode 214 of the pixels are connected to each other, and connected to a common voltage of the TFT-LCD.
As shown in FIG. 2C, each R, G, or B sub-pixel is controlled by the data line (DL) and the scan line (SL). Each sub-pixel comprises a thin film transistor (TFT) 27, a pixel electrode (PE) 204, and a common electrode (VCOM) of the storage capacitor. The common electrode (VCOM) of FIG. 2C is the patterned first metal layer (denoted as 214) of FIG. 2B. The patterned second metal layer is as capacitor electrode 218 and is formed above the common electrode (VCOM), and the pixel electrode 204 on the top is electrically connected to the capacitor electrode 218 through the contact hole 219. Also, several slits 220 are formed in the pixel electrode 204 to acquire a result of the multi-domains and wide viewing angle. Furthermore, a protrusion 108 formed on the first substrate structure 10 is also demonstrated in FIG. 2C, for being used as another structure to achieve multi-domains and wide viewing angle effect.
Although multi-domains and wide viewing angle effect of display can be achieved by forming the protrusions 108 (as shown in FIG. 1A and FIG. 1B) or/and the slits 220 (as shown in FIG. 2C), the protrusions 108 or/and the slits 220 can cause the considerable problem of light leakage in the dark-state. Typically, the conventional display is not completely dark when it is in a dark-state. Also, the differences of light-leaking amounts of the RGB sub-pixels cause the color-shifting problem. The conventional display usually occurs a color of dark with a tendency of blue (not completely dark) when it is in a dark-state.